Computer guided cryosurgery

ABSTRACT

The present invention is a system for placing cryoprobes in a treatment area of a human patient. The system includes a computer system for displaying an image of a treatment area, a template of suggested cryoprobe palcements in the treatment area and images of actual cryoprobes placed within treatment area. The computer system is programmed to perform the steps of: acquiring the image of the treatment area, determining desired dimensions of the treatment area based on the image of the treatment area, determining the optimal cryoprobe placements in the treatment area based on the determined desired dimensions to provide the template of suggested cryoprobe placement, acquiring the images of actual cryoprobes placed within the treatment area, and overlaying the template of suggested cyroprobe placements on the acquired images of actual cryoprobes. The resulting overlaying is provided on a display of the computer system, thereby allowing the surgeon to compare the actual cryoprobe palcements with the optimal cryoprobe placements as determined by the computer system.

CROSS REFERENCE

This application is a continuation of U.S. Ser. No. 09/699,938, filedOct. 30, 2000 which is a continuation of U.S. Pat. No. 6,139,544 issuedOct. 31, 2000. (U.S. Ser. No. 09/318,710, filed May 26, 1999.)

FIELD OF THE INVENTIONS

The inventions described herein relate to the field of cryosurgery andablative surgery.

BACKGROUND OF THE INVENTIONS

The system and methods described below enhance the accuracy andeffectiveness of cryosurgery of the prostate and other treatment areasof a human patient. Cryosurgery of prostate is an effective treatmentfor prostate cancer and benign prostate hyperplasia, conditions whichaffect many men.

The use of cryosurgical probes for cryoablation of prostate is describedin Onik, Ultrasound-Guided Crvosurgery, Scientific American at 62(January 1996) and Onik, Cohen, et al., Transrectal Ultrasound-GuidedPercutaneous Radial Cryosurgical Ablation Of The Prostate, 72 Cancer1291 (1993). In this procedure, generally referred to as cryoablation ofthe prostate, several cryosurgical probes are inserted through the skinin the perineal area (between the scrotum and the anus) which providesthe easiest access to the prostate. The probes are pushed into theprostate gland through previously placed cannulas. Placement of theprobes within the prostate gland is visualized with an ultrasoundimaging probe placed in the rectum. The probes are quickly cooled totemperatures typically below −120 C. The prostate tissue is killed bythe freezing, and any tumor or cancer within the prostate is alsokilled. The body will absorb some of the dead tissue over a period ofseveral weeks. Other necrosed tissue may slough off through the urethra.The urethra, bladder neck sphincter and external sphincter are protectedfrom freezing by a warming catheter placed in the urethra andcontinuously flushed with warm saline to keep the urethra from freezing.

To maximize the effectiveness of-the procedure, the entire prostateshould be ablated. At the same time, surrounding structures such as therectum and the neurovascular bundles should not be frozen. The amount ofthe prostate which is ablated by the cryosurgical procedure depends onthe number of cryoprobes used and their placement within the prostategland. Wong, et al., Cryosurgery as a Treatment for Prostate Carcinoma,79 Cancer 963 (March 1997), suggests a placement scheme for cryosurgicalprobes within the prostate. Probes were inserted through the perinealarea into the prostate while attempting to keep the probes within 1.8 cmof each other. The systems and methods presented below were developed toassist surgeons in placing the probes as suggested by Wong, or assuggested by others, with the assistance of ultrasound imaging andcomputer graphics and computer assisted calculations of optimal probeplacement within the prostate.

SUMMARY

The present invention is a system for placing cryoprobes in a treatmentarea of a human patient. The system includes a computer system fordisplaying an image of a treatment area, a template of suggestedcryoprobe placements in the treatment area and images of actualcryoprobes placed within the treatment area. The computer system isprogrammed to perform the steps of: acquiring the image of the treatmentarea, determining desired dimensions of the treatment area based on theimage of the treatment area, determining the optimal cryoprobeplacements in the treatment area based on the determined desireddimensions to provide the template of suggested cryoprobe placements,acquiring the images of actual cryoprobes placed within the treatmentarea, and overlaying the template of suggested cryoprobe placements onthe acquired images of actual cryoprobes. The resulting overlaying isprovided on a display of the computer system, thereby allowing thesurgeon to compare the actual cryoprobe placements with the optimalcryoprobe placements as determined by the computer system.

Also described are algorithms and a corresponding computer programdesigned to calculate the optimal position of the cryoprobes foreffective cryosurgical ablation of the prostate in a wide range ofpatients. The algorithms decide whether the prostate size fits withinparameters for successful calculations, whether five or sixth probes arerequired, the optimal placement of two cryoprobes in the anterior lobeof the prostate gland, the optimum placement for two cryoprobes in theouter portions of the posterior lobe of the prostate, and the optimumplacement for one or two cryoprobes in the center area of the posteriorlobe of the prostate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of the transperineal cryosurgical ablation of theprostate.

FIG. 2 is is an illustration of a sagittal or longitudinal cross sectionof the prostate and surrounding anatomy as displayed on the videodisplay provided by the system.

FIG. 3 is an illustration of a horizontal or transverse cross section ofthe prostate and surrounding anatomy as displayed on the video displayprovided by the system.

FIG. 4 is an illustration of the system in use to outline the prostateimage shown in FIG. 2.

FIG. 5 is an illustration of the system in use to outline the prostateimage shown in FIG. 2, illustrating measurements calculated by thecomputer system.

FIG. 6 is an illustration of the system in use to outline the prostateimage shown in FIG. 3.

FIG. 7 is an illustration of the system in use to outline the prostateimage shown in FIG. 3, illustrating measurements calculated by thecomputer system.

FIG. 8 is an illustration of the system determination of the optimumplacement of cryoprobes within the anterior lobe of the prostate imageshown in FIG. 3, where the prostate is relatively large.

FIG. 9 is an illustration of the system determination of the optimumplacement of two cryoprobes within the anterior lobe of the prostateimage shown in FIG. 3, where the prostate is relatively large.

FIG. 10 is an illustration of the system determination of the optimumplacement of two cryoprobes within the anterior lobe of the prostateimage shown in FIG. 3 where the prostate is relatively small.

FIG. 11 is an illustration of the system determination of the optimumplacement of two cryoprobes within the posterior lobe of the prostateimage shown in FIG. 3

FIG. 12 is an illustration of the system determination of the optimumplacement of a fifth cryoprobe within the prostate image shown in FIG.3, in the case that only five probes are required for effectivecryosurgical ablation of the prostate.

FIG. 13 is an illustration of the system determination of the optimumplacement of the fifth and sixth cryoprobes within the prostate imageshown in FIG. 3, in the case that a sixth probe is required foreffective cryosurgical ablation of the prostate.

FIGS. 14 through 21 illustrate a second method for the systemdetermination of the optimum placement of cryoprobes within theprostate.

FIG. 22 is an illustration of the system output indicating the optimumplacement of the fifth and sixth cryoprobes within the prostate imageshown in FIG. 3, in the case that a sixth probe is required foreffective cryosurgical ablation of the prostate.

FIG. 23 is an illustration of the system output indicating the optimumplacement of cryoprobes within the prostate image shown in FIG. 2.

FIG. 24 is an illustration of the system output indicating the optimumplacement of cryoprobes within the prostate image shown in FIG. 2

FIG. 25 is an illustration of the system output indicating the actualplacement of cryoprobes in relation to the displayed optimum placementof cryoprobes within the prostate image shown in FIG. 2.

FIG. 26 is an illustration of the system output indicating the actualplacement of cryoprobes in relation to the displayed optimum placementof cryoprobes within the prostate image shown in FIG. 2, displayingprobes and graphical markers obscured in the view of FIG. 25

FIG. 27 is an illustration of the system output indicating the actualplacement of cryoprobes in relation to the displayed optimum placementof cryoprobes within the prostate image shown in FIG. 3.

FIG. 28 is a copy of the portion of the computer program which performsthe calculations used to determine the optimum placement of cryoprobes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one of the basic operations for which the cryoprobes aredesigned. Several probes 1, 2, 3, 4 and 5 are shown inserted in theprostate 6 (in some instances a sixth probe, not shown, is used). Allfive probes are inserted through the perineal region 7 between thescrotum and the anus. Probes 2 and 3 are shown inserted into theanterior lobe 6a of the prostate, and Probes 1, 4 and 5 are showninserted into the posterior lobe 6p, which is larger than the anteriorlobe. The probes are placed within the prostate according to procedureswell known in the art, and a suitable procedure is described instep-by-step detail in Onik, et al., Percutaneous Prostate Cryoablation,(1995) at pages 108-112 and Onik, Ultrasound-Guided Cryosurgery,Scientific American at 62 (January 1996). Typically five or six probesare used in the procedure, though more or less may be used when requiredby unusual anatomy of a particular patient. The urethra 8 which passesthrough the prostate is one of the anatomic structures that usuallyshould not be frozen during this surgery. Accordingly, the urethra isprotected and kept warm with the urethral warming catheter 9. Thebladder neck sphincter 10 and the external sphincter 11 are alsostructures that should be protected from freezing, and these areprotected from freezing by the warming catheter. Neurovascular bundleson the right and left of the prostate should also be protected fromfreezing. Transrectal probe 13 is inserted into the rectum 14 in orderto visualize the placement of the probes and the growth of the iceballsformed by the cryoprobes. To assist in placement of the cryosurgicalprobes, a template 15 is used which supports the probes during insertionand while they are installed in the body. The patient is placedhorizontally on an operating table with legs positioned to provideaccess for the ultrasound probe to be inserted into the rectum andcryoprobes to be inserted through the perineal area into the prostate.

The transrectal ultrasound probe 13 is used to visualize the prostateand the cryosurgical probes. The ultrasound probe operates in the rangeof about 2-10 MHz, depending on the equipment used (the processdescribed herein may be used with any ultrasound probe and ultrasoundgenerator, which may be selected based on various technical, medical andbudgetary considerations). The image is displayed as a two dimensionalrepresentation of the boundaries of the prostate, as illustrated inFIGS. 2 and 3.

FIG. 2 shows a longitudinal (also referred to as saggital or coronal)cross section of the prostate, generally denoted by the ultrasoundoutline 21. The top, bottom, anterior and posterior sides of the outlineare delineated by the letters T, B, A and P, respectively. The apex ofthe prostate 22 points generally to the perineal area shown in FIG. 1.Other anatomical structures visible in display include the urethra,whose outline 23 may not appear fully in the coronal cross section. Theurethral warming catheter image 24 appears within the urethra. Also, theultrasound image of the rectal wall 25 appears along the bottom of thedisplay area, and the image of the ultrasound probe 26 may appeardirectly below the rectal wall. The rectum and rectal wall arepreferably straightened by the pressure exerted by the insertion of theultrasound probe, as explained below, to set up the system for thecalculations and displays described below. Since the prostate isgenerally oblong in this direction, and longer in this direction than inthe horizontal or transverse cross section, we refer to this crosssection as the longitudinal cross section.

FIG. 3 shows a horizontal or transverse cross section of the prostategenerally denoted by the ultrasound outline 28. The anterior (towardsthe front of the body), posterior (toward the back), right and left ofthe prostate are marked as A, P, R and L, respectively. The urethraappears as the ultrasound outline 29. The warming catheter outline 24appears within the urethra image 29. Also visualized in the display isthe shadow of the probe itself, marked as item 30, and the ultrasoundoutline of the rectum 31. The image shown is refreshed at a regular rateby the ultrasound imaging system and the images may shift with movementof the ultrasound probe. The surgeon is instructed to translate theultrasound probe within the rectum to obtain several cross sectionalviews and to choose the largest viewable cross section of the prostatefor analysis and display.

Other aspects of the display shown in FIGS. 2 and 3 are provided as partof the user interface. The ultrasound display area 32 is used by thesystem to display the image of the prostate generated by the ultrasoundimaging system, and is used as described below to enter informationregarding the prostate outline and display suggested probe placements.The toolbar area 33 across the top of the screen is used by the systemto present drawing and viewing tools and other interface tools (typicalselection, pen and zoom tool icons 34, 35 and 36 are illustrated in thetoolbar). The data presentation area 37 along the right side of thescreen is used by the system to indicate program information and patientinformation, step by step instructions to the surgeon, and measurementsderived from operator inputs in the display area.

The system provides a function for the operator to freeze the image inorder to accept outlining and path-finding inputs followed bycalculating functions. The operator is prompted to orient the ultrasoundimage such that the rectal wall is substantially parallel to the bottomedge of the display area. The operator is instructed or prompted tosearch for the largest cross section of the prostate as seen through theultrasound probe, and to freeze the image on the screen. In response tothe operator's instruction to the system to freeze the images, a singleframe will be captured or grabbed by the system software and displayedas a frozen image. The frozen images are presented on the display toallow the operator to interact with the system to determine the size andshape of the prostate.

FIG. 4 shows the grabbed image of the coronal cross section of theprostate shown in FIG. 2. First viewing the live image of FIG. 2 toinspect a number of cross sections along different planes, the surgeoninstructs the system to freeze the image when an appropriate crosssection is displayed. Generally, the largest visualized cross section inwhich the urethra appears should be used. Visualization of the urethraindicates that the image plan is at or near the center of the prostate.When instructed do so, the computer system grabs a frame from the liveimage, and presents a frozen image as illustrated in FIG. 4.

To determine if the cross section is acceptable and that the ultrasoundprobe is placed in the preferred position, the operator marks at leasttwo points 39 and 40 along the rectal wall image 25 and instructs thecomputer system to analyze these two points to make sure that they aresufficiently horizontal (parallel with the ultrasound probe) andstraight to support later functions of the system. If the rectal wallimage is not horizontal within acceptable limits (currently, not morethan 4 mm difference in height on the display area), and near the bottomof the display area within acceptable limits (currently set at 3 cm),the system will reject the frozen image as a basis for later functionsand prompt the operator to correct the image orientation, probeplacement or patient position. If the rectal wall image is substantiallyhorizontal on the display, indicating that the rectal wall is parallelwith the ultrasound probe, the system will indicate that the image isacceptable and prompt the operator to continue the procedure and outlinethe prostate image on the display.

Using outline functions of the supporting computer system, such as thepen tool selected from the toolbar, the operator outlines the image ofthe prostate by moving the pen tool around the outline and anchoring thepen tool at selected points around the perimeter until the prostate isfully outlined. (The anchor points may be created by the operator withstandard input devices such as a mouse or touch screen and otherstandard software tools such as an anchor point adding tool, pen tool orpolygon tool.) The operator then creates an outline of the horizontalcross section of the prostate by drawing a polygon having several anchorpoints 41, 42, 43, 44, 45, 46, and 47 around the perimeter of theprostate ultrasound image 21 (any number of anchor points may be used).The operator indicates to the system that the outline is accuraterelative to the displayed image, and the computer system accepts theimage as a representation of the coronal cross section of the prostate.

Referring to FIG. 5, the computer system searches the display data forthe outline and defines the two parameters H2 and L2. The parameter H2is the “height” of the prostate that will be considered by the system.The parameter L2 is the length of the prostate that will be consideredby the program. (It should be appreciated that any variable name may beassigned to these parameters; the H2 and L2 designations correspond tovariables used in the computer program which the inventors have devisedto implement the system.) At this point, the computer system analyzesthe variables to assist the operator in deciding how to accomplish thecryosurgery. If L2 is greater than 35 mm, the system will notify theoperator that a pullback freeze is required to completely ablate theprostate. The doctor will then be apprised that a single freezingoperation will be insufficient, and that the cryosurgery must beaccomplished in two steps, with a first freeze being accomplished withthe probe tips near the top of the prostate and the second freeze beaccomplished afterward, with the cryoprobes pulled back about 10 mmtoward the apex of the prostate. If H2 is less than a predetermineddistance from either end of the prostate, the system will prompt theoperator to verify that the outline accurately reflects the size andshape of the prostate, whereupon the process may continue or berestarted.

Next, the operator performs the outlining task in relation to thehorizontal cross section. Thus, FIG. 6 shows the display of FIG. 3modified by the operator with the addition of a polygon comprisingseveral anchor points 48, 49, 50, 51, 52, 53 and 54 around the perimeterof the prostate ultrasound image 28 (any number of points may beentered). The operator indicates to the system that the outline isaccurate relative to the displayed image, and the computer systemaccepts the image as a representation of the coronal cross section ofthe prostate. The operator is also prompted to mark the center of theurethral prostate, and enters a mark 55, which the computer systemaccepts as the location of the prostatic urethra in the horizontal crosssection.

As shown in FIG. 7, the computer system searches the display data forthe outline and defines the two parameters H1 and L1. The parameter H1is the “height” of the prostate that will be considered by the system,as determined in the horizontal cross section image. The parameter L1 isthe width of the prostate that will be considered by the program. Afterthe parameter H1 is determined, the computer system will compare it withthe parameter H2 (the height as determined in the coronal cross sectionimage as shown in FIG. 5). These two parameter are expected to be thesame if the operator has actually selected the largest identifiablecross section in both planes. If H1 is different from H2 by more that apredetermined amount, this indicates that the cross sections do notclosely correspond to the largest actual cross section, and the computersystem will prompt the operator to restart the procedure at the imageselection stage (presently, 3 mm difference is the maximum alloweddifference, but this may vary with experience with the present system,or with the cryoprobes in use). Upon determining L1, the computer willcompare it with a predetermined width, and if L1 is greater than thepredetermined width (currently set at 3.5 cm) the system will notify theoperator that six cryoprobes are required for the operation, and proceedas described below to calculate a position for the sixth cryoprobe.

The system will not calculate positions for extremely large or smallglands, thus if the prostate width in the anterior lobe is greater than54 mm, the system will display a message in the instruction area thatthe gland is too large for calculated positioning of the probes. If theprostate width in the anterior lobe is smaller than 23 mm, the systemwill display a message in the instructions area that the gland is toosmall for calculated positioning of the probes.

The supporting computer system is programmed with software which permitsthe drawing and outlining functions described above, and calculationfunctions for calculating the size of the prostate based on the input,determining the optimal number of probes required, and determining theoptimal placement for the probes. When the outline of the prostate hasbeen entered into the computer system, and the computer system haschecked that the various parameters are within predetermined ranges, thecomputer system operates to calculate the optimum number of cryoprobesand the optimum placement of the cryoprobes, and communicate thisinformation to the operator.

The computer system requires input or calculation of certain parametersin order to determine probe placement. The parameters required are:

H1: Maximum front to back thickness in the horizontal cross section;

L1: Maximum right to left thickness in the horizontal cross section;

H2: Maximum front to back thickness in the saggital cross section;

L2: Maximum right to left thickness in the saggital cross section;

prtUrethra: The location of the center of urethra within the horizontalcross section;

iYO: the vertical position of the anterior extreme of the prostate glandin the horizontal cross section;

iYO1 and iY5: The vertical position of the rectal wall in the horizontalcross section, as approximated by the posterior extreme of the prostategland.

The measured parameters are used to calculate the optimum position forthe placement of the cryosurgical probes. The optimum placement ofprobes is determined empirically, based upon histological evidence oflethal ablation zones, and the calculations may vary as histologicaldata accumulates. The system first calculates the position of probes inthe horizontal plane, and from this derives the position of the probesin the coronal cross section.

Using data from the outlining step, the system first calculates thedesired position of probes 2 and 3, which are placed in the interiorlobe of the prostate. Referring to FIG. 8 (horizontal cross section), ifL1 exceeds 3.6 cm, the computer system searches the horizontal outlineof the prostate (using a line by line measurement of the display datagenerated by the computer system) from the top to identify the firsthorizontal length which equals 3.6 cm. When this line is found, asexemplified by line 57, the line is divided into four sections of equallength, and the five points P1, P2, P3, P4, and P5. The system nexttests whether the distance from point P2 to the midline highest point 58(pctrpt) is less than 9 mm. If this distance is less than 9 mm, thesystem uses this vertical position for placement of the probes 2 and 3.Graphical markers corresponding to Probes 2 and 3 are placed on thepoints P2 and P4, which correspond to the middle points on the two linesegments from the outer edge of the prostate to the center of theprostate at the point where the width of the prostate first measures 3.6cm. If this distance is initially more than 9 mm, the system moves theline 57 upward until the distance is 9 mm, and adjusts P1 through P5 tomatch the outer edge of the prostate, the centerline of the prostate,and the points between the center and the outer edges, and uses this newvertical position for placement of probes 2 and 3. This is illustratedin FIG. 9, where line 57 a has been raised to a higher level in thedisplay so that distances 59 and 60 are 9 mm. Graphical markerscorresponding to Probes 2 and 3 are placed on the points P2 and P4,which now correspond to the middle points on the two line segments fromthe outer edge of the prostate to the center of the prostate at thepoint where the width of the prostate first measures 3.6 cm, after thisline segment has been raised to the point where the middle points are 9mm from the centerline front of the prostate. Thus, in the case wherethe prostate width is greater than 3.5 cm, the first two graphicalmarker probes are placed by the system on the display in optimalposition.

In the case that L1 is less than 3.5 cm, placement of the first twoprobes is determined as illustrated in FIG. 10. The computer systemsearches the horizontal outline of the prostate (using a line by linemeasurement of the display data generated by the computer system) byiteratively drawing horizontal lines and identifying the five points P1through P5, from the top to identify the first horizontal line in whichthe distance point P2 to the top centerline is 9 mm, or until thedistance between P2 and P4 is 9 mm, whichever occurs first. When thisline is found, as exemplified by line 61, the points P2 and P4 areidentified as the position for Probes 2 and 3, respectively.

With the positions for Probes 2 and 3 determined, the system nextdetermines the optimal position for probes 1 and 4, which are placed inthe posterior lobe of the prostate. The procedure is explained inreference to FIG. 11. The system finds all points at which the distancealong the horizontal from the outer edge of the prostate (distance b) isone half of the distance along the vertical from the outer edge of theprostate (distance a). All the points along lines 63 and 64 (bothrepresenting line =½a on opposite sides of the prostate) illustrated inFIG. 11 fit this criterion (note that the line is not necessarilystraight, since it is defined in reference to the irregular outline ofthe prostate). The system next draws the arc 65 at a distance of 18 mmfrom point Probe 2, and a similar arc 66 from point Probe 3. The pointwhere the arc 65 and the line 63 intersect is chosen as the position ofprobe 1 and the point where the arc 66 and the line 64 intersect ischosen as the position of probe 4. Currently, these probes arepositioned in this manner regardless of prostate size, except asinherited from the positioning of probes 2 and 3.

Placement of probe 5 depends on the size of the prostate. If the systemhas decided that 5 probes are sufficient to accomplish effectiveablation, the fifth probe is placed on the line H1 between the rectalwall and the urethra. The system finds the point along H1 which is ⅔ ofthe way up from the rectal wall, and assigns this position to Probe 5,as illustrated in FIG. 12. If the system has decided that 6 probes arenecessary (i.e., L1 is greater than 3.6 cm), the system determines theposition of probes 5 and 6. As illustrated in FIG. 13, the systemdetermines the position of these probes relative to previously placedprobes. The system determines the position of probe 6 so that it fallswithin 1.8 cm of probe 1, and the distance from the horizontalcenterline L1 to the probe is not more than 1 cm, and the distancebetween from the vertical center line H1 is not more that 9 mm. Arc 67is drawn around Probe 1 with radius of 18 mm, and a line 68 is drawn 1cm below L1. Probe 5 is placed on the higher of (1) the intersection ofline 69 (drawn 9 mm away from H1) with the arc 67, or (2) theintersection of the line 69 and line 68. A similar construction isperformed on the other side of the prostate, and probe 6 is placedwithin the intersection of line 70 and the arc 71 or the line 68,whichever is higher. Thus Probes 5 and 6 are located on the screenrelative to the ultrasound image of the prostate.

FIGS. 14 through 21 illustrate a second method for the systemdetermination of the optimum placement of cryoprobes within theprostate. Referring to FIG. 14, the prostate outline is used as beforeto define H1 and L1. The system finds the top of the prostate outline,and sets the variable iYO at the top of prostate (represented by thehorizontal line iYO), and sets the variable pctrpt as the x and y valuesat the top of the prostate. Minimum and maximum potential verticallevels are defined for analysis as locations for probes 2 and 3. Thesystem defines the variable ip2ymin at 8 mm below the line iYO (labeledas horizontal line ip2ymin), and defines the variable ip2ymax at highestof (1) 16 mm below iYO (labeled as horizontal line iYO+16 mm) and (2) 4mm above the center of the urethra (labeled as horizontal linepctUrethra-4 mm), and defines the variable ip2ylimit (the lowest allowedheight for probes 2 and 3) as the horizontal line {fraction (7/16)} downH1 (labeled iP2ylimit). The system searches line on the screen startingat ip2ymin and continuing through to the highest of ip2ymax orip2ylimit, which ever is higher. (If iP2ymin is lower than iP2ymax, thesystem sets ip2ymin as the y value for probes 2 and 3.)

Referring now to FIG. 15, the system searches the region defined in FIG.14 for the desired vertical placement of probes 2 and 3. The systemdetermines the length of ip2ymin, and the length of line ip2ymax acrossthe prostate. If upper line (iP2ymin) length (dpt1topt2min) is greaterthan 54 mm, the outline cannot be automatically analyzed because it istoo big, and the system communicates this to the operator. If the lowerline (iP2ymax) length (dpt1topt2max) is smaller than 23 mm, the outlinecannot be automatically analyzed because it is too small, and the systemcommunicates this to the operator. In all other instances, the systemcontinues on to create and define variables as follows:

For L1 between 26 and 36 mm, create variable pt1topt2, and store theminimum value of 26 or dpt1topt2max (length of ip2ymax line) aspt1topt2;

For L1 between 36 and 54 mm, store the minimum of 36 or dpt1topt2max(length of line iP2ymax) as pt1topt2;

For L1 between 54 and 58 mm, store the minimum of 44 or dpt1topt2max aspt1topt2;

For L1 greater than 58 mm, store the minimum of 54 or dpt1topt2max aspt1topt2.

Now referring to FIG. 16, the system searches for the proper height forprobes 2 and 3. From the top line (iP2ymin), the system scans each pixelline from iP2ymin to iP2ymax, searching for outside points X1 and X5 onthe outline intersecting the horizontal line. When a line is found wherethe length of this line equals the defined dPt1toPt2 (calculated inreference to FIG. 15, above), that line is used as the verticalplacement of probes 2 and 3. The corresponding line in FIG. 16 islabeled 72. Finally, the system calculates the x values for probes 2 and3, setting them along the line at a spacing of ¼ and ¾ from the left,respectively. Thus, the system determines the position of two probes inthe anterior lobe of the prostate based upon the width of the prostateapproximately at the anterior-most location of (1) 16 mm posterior tothe anterior extremity of the prostate, (2) 4 mm anterior to the centerof the prostatic urethra, or (3) {fraction (7/16)} of the totalthickness of the prostate from the anterior extremity of the prostate.

Now referring to FIG. 17, the system analyzes the outline to determinethe proper location of probes 1 and 4 in the posterior lobe of theprostate. The system defines the vertical location iY01 (horizontal lineiY01 in FIG. 17) as the y value or line at ⅝ down the prostate (⅝ downthe length of H1). The system determines the Y distance between iY01line and probes 2 and 3 (about 10 mm, as illustrated). If this distanceis less than 16 mm, the system uses iY01 line as vertical height forplacement of probes 1 and 4. Here, iY01 is only 10 mm away, so this isheight for probes 1 and 4. If the distance is greater than 16 mm, thesystem raises line iY01 until it is only 16 mm down from probes 2 and 3.To determine the horizontal placement of probes 1 and 4, the systemplaces point iXL ⅙ across iY01 line, and if greater than 18 mm fromprobe 2, slide right (or left) until the point is 18 mm from probe 2(i.e., within circle 73, drawn at a radius of 18 mm from probe 2).Similarly, the system places point iXR ⅚ of the distance across the lineiY01, and if greater than 18 mm from probe 3, the point is moved left(or right) until the point is 18 mm from probe 3 (i.e., within circle74, drawn at a radius of 18 mm from probe 3). As illustrated, bothpoints fall within 18 mm of the probe above, so points iXL and iXR areused as the points for suggested placement of probes 1 and 4respectively. FIG. 18 illustrates the same procedure in relation to alarger prostate which requires sliding point iXR to the left afterinitial placement at ⅚ across line iY01, so that it comes within 18 mmof probe 3 (and meets circle 74). In all other respects, FIG. 18 isdescribed in the same manner as FIG. 17. Thus, the system determines theposition of two probes in the posterior lobe of the prostate based uponthe width of the prostate at the anterior-most location of (1)approximately ⅝ of the total thickness of the prostate from the anteriorextremity of the prostate or (2) 16 mm posterior to the two probespositioned in the anterior lobe of the prostate.

Next, the system determines the placement of Probes 5 and 6 within theposterior lobe of the prostate. Referring to FIG. 19, the system setsline iY01 as y height of probes 1 and 4, and sets the horizontal lineiY56 at ¼ up from bottom of screen (prostate/rectum/screen bottom are inreality so close together that they may be treated as the same y level),and sets the line (labeled as the horizontal line iY01+16)16 mm downfrom probes 1 and 4. Then, the system resets line iY56 at the highest(the minimum y value) of the two lines, here the ¼ up line is chosen.The system places points X5 and X6 at left and right extremities of lineiY56, which is the intersection of the prostate outline with iY56. Toplace the probes 5 and 6 at the appropriate horizontal location, thesystem moves points X5 and X6 inward toward H1 until they are 10 mmapart. FIG. 20 illustrates the placement of probes 5 and 6 in a largerprostate, to illustrate the system choice of the highest line forplacement of the probes. Here, line iY56 is below line iY01+16, so lineiY56 is reset to the height of line iY01+16. Once again, the points X5and X6 are moved inward along the higher line until they are only 10 mmapart, and probes 5 and 6 are located at these points, as illustrated inthe figure. Thus the system determines the position of the two probes 5and 6 in the center portion of the posterior lobe of the prostate basedupon the width of the prostate at a the anterior-most location of (1)approximately ¼ of the distance on the display from the rectum to thetop of the display; (2) 16 mm posterior to the two probes 1 and 4already positioned in the posterior lobe of the prostate.

In a prostate where L1 is less that 35 mm across, only probe 5 is used,and it is located at or near the horizontal center of the prostateoutline. As illustrated in FIG. 21, the system sets iY5 at bottom ofoutline. The system also defines the horizontal line ⅓ the way down H1from the urethra to rectum (the horizontal line labeled aspctUrethra+⅓), and defines the horizontal line 18 mm above iY5 (thehorizontal line labeled iY5-18). The system then selects the lowest ofthese two lines as the vertical position for probe 5. Probe 5 is placedhorizontally on the horizontal midpoint between probes 1 and 4 (expectedto be on or near H1). Thus, as an alternate to placement of both probes5 and 6, the system places probe 5, based upon the width of theprostate, in the center portion of the prostate at the posterior-mostlocation of (1) ⅓ the distance posterior from the urethra to the rectumor (2) 18 mm anterior to the posterior extremity of the prostate.

After calculation of the optimal placement of the cryoprobes accordingto either of the methods above, the system graphically displays thedesired locations to assist the operator in placing the actual probes inthe prostate of the patient. The optimal location of the probes isindicated in the horizontal cross section by a graphic representationoverlaid over the live ultrasound images of FIGS. 3 and 2 and/or thestill images of FIGS. 6 and 4. The desired representation is illustratedin FIGS. 22 and 25. FIG. 22 is an illustration of the system outputindicating the optimum placement of cryoprobes within the prostatehorizontal cross sectional image shown in FIG. 2. The suggested probeplacement is indicated by graphical markers 75, 76, 77, 78, 79 and 80for Probes 1, 2, 3 4, 5 and 6 respectively. The markers are placed inthe display by the computer system, overlaying the ultrasound image ofthe prostate horizontal cross section.

The system also provides an illustration of the placement of the probesin the sagittal cross section. FIGS. 23 and 24 show the displaypresented by the computer system of graphical markers for each probe.The vertical position of the probes is dependent on the positionscalculated in reference to calculated positions in the horizontal crosssection. FIGS. 23 and 24 show the system output indicating the optimumplacement of cryoprobes within the prostate longitudinal cross sectionalimage shown in FIG. 3. The suggested probe placement, as viewed on thesaggital plane, is indicated by graphical markers, 81, 82, 83, 84, 85and 86. The markers are placed in the display by the computer system,overlaying the ultrasound image of the prostate saggital cross section.Since probes 2 and 3, and probes 1 and 4, and probes 5 and 6 are likelyto be symmetrically located in reference to the line H1, the probes inthese pairs will overlap in this view. Thus, probes 3, 4 and 6 aredisplayed as shown in FIG. 24. The computer system permits the operatorto switch between views of FIG. 23 and FIG. 24. FIG. 23 is used by theoperator to assist in placement of Probes 1, 2 and 5, while FIG. 24 isused to assist in placement of Probes 3, 4 and 6.

With the optimal probe placements calculated and graphical markersplaced on the display, the operator may insert cryoprobes into theprostate. FIGS. 25, 26 and 27 illustrate the feedback provided to theoperator indicating the actual position of the cryoprobes in relation tothe suggested placement shown in FIGS. 22, 23 and 24. Referring to FIG.27, the graphical markers 75, 76, 77, 78, and 80 are shown in thehorizontal cross section, displayed as generated by the computer system.In addition, the ultrasound image of the probes is displayed in thedisplay area, since the probes enter the ultrasound imaging field andare imaged by the ultrasound imaging system. As the surgeon inserts eachprobe into the prostate, its placement as indicated by the ultrasoundsystem may be compared to the suggested probe placement, and the surgeonmay manipulate the probes so that the ultrasound images of the actualprobes align with the graphical markers. In FIG. 27, the exemplaryplacement of probes is illustrated. Probe 1 has been placed in goodcorrespondence with the template provided by the computer, and theultrasound image is aligned with the graphical marker 75. Probe 2 hasbeen placed in a position different than its associated graphical marker76, and the surgeon may decide on that basis to reinsert the probe tomore closely align it with the marker. Probe 3 has been placed close tothe marker 77, and the surgeon may decide to reposition the probe or toleave it in place. Likewise, Probes 4 and 5 appear in the display on ornear their associated markers 78 and 80, providing feedback to thesurgeon ensuring proper placement of the probes.

Referring to FIG. 25, the graphical markers 81, 82 and 85 are shown,displayed as generated by the computer system. In addition, theultrasound image of the probes is displayed in the display area, sincethe probes enter the ultrasound imaging field and are imaged by theultrasound imaging system. Again, the surgeon may view the ultrasoundimage of the actual probes, and place the probes as closely as possiblepositions corresponding to the markers. FIG. 26 shows a displaycontaining the graphical markers 83 and 84 corresponding to probes 3 and4 which are located on the right side of the prostate relative to theanterior/posterior centerline. These displays help the operator inplacing the probes as desired in parallel relationship with theultrasound probe and the rectal wall. The operator may switch repeatedlybeing the displays of FIGS. 25, 26 and 27 while inserting thecryoprobes, selectively displaying the image of the horizontal crosssection and the image of the coronal cross section, to monitor theprogress of the probes and ensure placement of the probes isaccomplished in the positions suggested by the computer system. Whencryoprobe placement is satisfactory, the surgeon will start the flow ofcooling gas to freeze the prostate. The freezing operation can beconfirmed in the ultrasound image by watching the iceballs (the mass offrozen tissue) around each cryoprobe form. The extent of the iceballsand the extent of the prostate that is frozen is monitored to ensurethat substantially all of the prostate is frozen. The freezing processmay be repeated to ensure ablation of the prostate.

The section of the computer program which performs these calculations isprovided as FIG. 28, which is programmed in the C++programming language.The program implements the second method described above in relation toFIGS. 14 through 22. The section of the code which finds the propervertical location for probes 2 and 3 in the anterior lobe of theprostate starts at item number 90. The section of code which determinesif the gland is too large or small is indicated by item number 91. Thesection of the code which finds the proper horizontal and verticallocation for probes 2 and 3 in the anterior lobe of the prostate islabeled as item 92. The section of the program which calculates theposition of probes 1 and 4 is indicated by item number 93. This segmentof code also incorporates a test for adequate symmetry of the prostate,in that if probe 1 or probe 4 cannot be properly place, the systemcommunicates this to the operator (leading to re-imaging or cancellationof the computer assisted surgery). The section of the program whichcalculates the position of probes 5 and 6 (or only probe 5) is indicatedby item number 94.

Thus, we have described a system for assisting surgeons in performingcryosurgery of the prostate by calculating optimal positions forcryoprobes and providing display based templates for overlay over anultrasound image display, and displaying actual cryoprobe ultrasoundimages together with template images so that the surgeon may comparesuggested and actual placement of the probes, and adjust placementaccordingly. The method and system is described above in relation to ourCRYOcare™ cryosurgical system, which is provided with up to eightindependently controlled 3 mm argon powered cryoprobes. The system coolsthe probes to cryosurgically effective temperatures (typically below−120° C.) through Joule-Thomson cooling within the probe tips. Thesystem may be implemented with other cooling systems such as liquidnitrogen cryoprobes and mixed gas cryoprobes. The placement of probes iscalculated based on this system, and the calculations may be adjustedfor different systems and numbers of probes. Additionally, while thesystem has been described with ultrasound as the imaging mechanism andthe rectum as the point of view, the system may be implemented withother imaging systems such as fluoroscopy or new imaging systems. Thesystem may be adapted to other forms of ablation and treatment of theprostate or other organs, with adjustments in the calculations beingmade to account for the ablative range of the devices used and thegeometry of the organ. Thus, while the preferred embodiments of thedevices and methods have been described in reference to the environmentin which they were developed, they are merely illustrative of theprinciples of the inventions. Other embodiments and configurations maybe devised without departing from the spirit of the inventions and thescope of the appended claims.

What is claimed is:
 1. A system for placing cryoprobes in a treatmentarea of a human patient, comprising: a computer system for displaying:an image of a treatment area; a template of suggested cryoprobeplacements in the treatment area; and, images of actual cryoprobesplaced within the treatment area, said computer system being programmedto perform the steps of: acquiring the image of the treatment area;determining desired dimensions of the treatment area based on the imageof the treatment area; determining the optimal cryoprobe placements inthe treatment area based on said determined desired dimensions toprovide the template of suggested cryoprobe placements; acquiring theimages of actual cryoprobes placed within the treatment area; and,overlaying said template of suggested cryoprobe placements on saidacquired images of actual cryoprobes, the resulting overlaying beingprovided on a display of said computer system, thereby allowing thesurgeon to compare the actual cryoprobe placements with the optimalcryoprobe placements as determined by said computer system.
 2. Thesystem of claim 1, wherein said computer system acquires an image of atreatment area from the image data of a desired imaging device.
 3. Thesystem of claim 2, wherein said desired imaging device comprises anultrasound device.
 4. The system of claim 2, wherein said desiredimaging device comprises a fluoroscopic imaging device.
 5. The system ofclaim 1, wherein said step of acquiring an image of the treatment areacomprises acquiring an image of a prostate.
 6. The system of claim 5,wherein said step of determining the desired dimensions of the treatmentarea comprises determining the width and height of the prostate.
 7. Thesystem of claim 6, wherein said step of determining said optimalcryoprobe placements comprises the steps of: determining the optimalposition of two cryoprobes in an anterior lobe of the prostate;determining the optimal position of two cryoprobes in a posterior lobeof the prostate; and, determining the optimal position of two cryoprobesin a center portion of the posterior lobe of the prostate, or,alternately, determining the optimal position of a single cryoprobe in acenter portion of the prostate.
 8. The system of claim 7, wherein saidstep of determining the optimal position of two cryoprobes in ananterior lobe of the prostate comprises determining said optimalposition of two cryoprobes in the anterior lobe of the prostate basedupon the width of the prostate approximately at the anterior-mostlocation of (1) 16 mm posterior to the anterior extremity of theprostate, (2) 4 mm anterior to the center of the prostatic urethra, or(3) {fraction (7/16)} of the total thickness of the prostate from theanterior extremity of the prostate.
 9. The system of claim 7, whereinsaid step of determining the optimal position of two cryoprobes in aposterior lobe of the prostate comprises determining the optimalposition of two cryoprobes in the posterior lobe of the prostate basedupon the width of the prostate at the anterior-most location of (1)approximately ⅝ of the total thickness of the prostate from the anteriorextremity of the prostate or (2) 16 mm posterior to the two cryoprobespositioned in the anterior lobe of the prostate.
 10. The system of claim7, wherein said step of determining the optimal position of twocryoprobes in a center portion of the posterior lobe of the prostate,or, alternately, determining the optimal position of a single cryoprobein a center portion of the prostate, comprises: determining the optimalposition of two cryoprobes in the center portion of the posterior lobeof the prostate based upon the width of the prostate at a theanterior-most location of (1) approximately ¼ of the distance on thedisplay from the rectum to the top of the display or (2) 16 mm posteriorto the two cryoprobes positioned in the posterior lobe of the prostate,or, alternately, based upon the width of the prostate, determining theoptimal position of a single cryoprobe in the center portion of theprostate at the posterior-most location of (1) ⅓ the distance posteriorfrom the urethra to the rectum or (2) 18 mm anterior to the posteriorextremity of the prostate.